Subterranean wells are generally provided with one or more downhole safety valves. The primary function of the safety valve is to provide a reliable barrier between the well stream and the production control system in the event of the loss of pressure or structural integrity of the production system. Such a loss might include a catastrophic failure of the wellhead.
A conventional downhole safety valve is operated from the surface of the well through a platform supplied hydraulic control line strapped to an external surface of the production tubing. The valve control operates in a fail safe mode; that is hydraulic control pressure is used to hold open a ball or flapper assembly that will close if the control pressure is lost.
Downhole safety valves are routinely tested to ensure they will close in the event of a catastrophe. However, the valves sometimes close during normal production operations. One of the primary causes of valve closure is failure of the hydraulic control system. Safety valves are normally biased to the closed position by wellhead pressure, which can exceed 10000 psi. The pressure required to retain the valve in the open configuration is higher, normally equating to maximum wellhead pressure plus 1500 psi. Such a pressure requirement can be difficult to maintain, especially over the large distances encountered in an oil well. This problem is exacerbated in higher pressure wells.
When a downhole safety valve fails, production stops generally necessitating an unscheduled workover, which requires the use of a vessel. Not only is this an expensive operation but it also prevents health and safety risk, particularly in a deep water environment.